An implantable medical device (IMD), such as a pacemaker and/or implantable cardioverter-defibrillator (ICD), regulates or synchronizes the beating of the heart with electrical impulses, delivered by electrical leads having electrodes contacting the heart muscles. Some IMDs include a number of different sensors and logic allowing them to monitor the rate and rhythm of the heart as well as to measure various cardiac surrogates that provide information on the operation of the heart.
Cardiac resynchronization therapy (CRT) often employs a left ventricular lead placed via the coronary sinus. The electrical lead itself is typically small and has some curvature to achieve passive fixation, i.e., not actually anchoring into the myocardium. Rather than relying on anchoring, the shape of the lead attempts to hold the lead in place. Due to movement of the patient and also cardiac motion (even when the patient is not moving) the lead may dislodge from the location where the physician had implanted it. Moreover, in some lead implant sites, e.g., in the coronary venous system of the heart, counter flowing blood could also promote dislodgement.
Dislodgement of pacing leads, particularly the passively-fixated left ventricle leads in the coronary sinus, is viewed as a risk associated with the CRT implant procedure. Sometimes due to patient anatomy and/or limitations of leads and delivery tools, it is difficult to place the lead in a stable location that avoids phrenic nerve stimulation and that provides adequate cardiac resynchronization when paced. Patient posture in the hours following implant can exacerbate problems with lead stability. Even between the subacute period and to several weeks post-procedure, the course of time, in which fibrosis/tissue in-growth “locks” the lead in place, is not well known. Finally, with cardiac remodeling due to progressively worsening heart failure or due to reverse remodeling from efficacious therapies, it is possible for a lead to migrate with time.
Although impedance can be used to detect lead dislodgment, impedance is not entirely reliable. For example, impendence varies over the first several days after implant, as well as when the lead matures. Moreover, if the lead stabilizes in a new location, then the impedance may not appear to change substantially, thus, hindering detection of lead migration.
Fluoroscopic procedures can detect a macro dislodgement (e.g., five to ten millimeters) but such procedures expose a patient and the physician to further radiation through the fluoroscope x-rays. Moreover, fluoroscopy may not detect micro dislodgment.